1. Field of the Invention
The present invention is directed to a polarized electromagnet, and more particularly to an improvement on a polarized electromagnet with an axially movable core which extends through an excitation coil and which has pole plates at its opposite ends respectively located between magnetic gaps formed between the opposite poles of permanent magnet means for axial movement upon energization and deenergization of the excitation coil.
2. Description of the Prior Art
Such polarized electromagnet is well known in the art. For example, U.S. Pat. No. 4,509,026 discloses a general structure of the polarized electromagnet in which an axially movable core extends through an excitation coil with pole plates at the opposite ends of the core located in magnetic gaps formed between the opposed pole ends of inner and outer yoke means which are magnetized by permanent magnet means to opposite polarities. In this structure, the inner yoke means have its pole ends extending inwardly of the lateral ends of pole plates in abuttable relation therewith so that the pole plate comes into engagement with the adjacent pole end of the inner yoke means when attracted thereto. This means that each pole end of the inner yoke means is spaced axially from the adjacent pole plate being in the position magnetically repelled from the pole end by a distance exactly equal to the stroke required for the axial movement of the core. Thus, in order to move the pole plate towards the adjacent pole end of the inner yoke means by the energization of the coil it is always required to generate a magnetic force compensating for the magnetic resistance determined by that distance. In other words, the prior electromagnet has response sensitivity inherently and directly determined by that distance between the pole end of the inner yoke means and the adjacent pole plate in the position repelled away therefrom, and is therefore practically impossible to raise the response sensitivity without reducing that distance or the stroke of the core.